Two primary methods are used for preparing silane-modified fillers such as are used, for example, in the plastics or rubber industry. In one method, organically modified alkoxysilanes Y—R—Si(R′x(OR″)3-x are added to fillers to prepare a crude mixture for reinforcing rubber mixtures (U.S. Pat. No. 3,997,356). In a second method, preparations are made from a preformed mixture of organically modified alkoxysilane and filler (DE 3314742, U.S. Pat. No. 4,076,550).
One disadvantage of these mixtures of alkoxysilane and filler is their lack of stability. During storage, small amounts of alcohol are continuously liberated by hydrolysis and condensation of the alkoxysilanes. The material composition and the performance of the filler/silane mixtures change as a result.
EP 1,256,604 discloses a process for the reaction of at least one biopolymeric, biooligomeric, oxidic or silicatic filler in a compressed gas with at least one silane. A high throughput rate of compressed gas is desirable in the process because it is thereby possible to shorten the necessary extraction time or to increase the amount of substance extracted per unit time. In EP 1,256,604, the compressed gas flows axially through the fillers undergoing extraction, or the filters are stirred in a pressure container. It can be seen from the examples that the axial flow of supercritical CO2 through the filler depends greatly on the particle size distribution of the fillers and on the length of the autoclave container used. For economic reasons, pressure autoclaves, particularly those used in large production installations, are usually designed as slim, long containers.
A disadvantage of the procedure is that, at the desired high throughput rates of compressed gases, high differences in pressure occur between gas entry and gas exit in the extractions, and this can damage components of the installation (in particular the sintered plates which retain the extraction material). There is also the risk of the formation of channels, with the consequence of a non-uniform flow through the bulk material and, as a result, a non-uniform extraction of the fillers. If only low throughput rates of compressed gases are used in order to keep the differences in pressure low, this leads to long extraction times. In the specific case of finely divided pulverulent fillers, only unsatisfactory throughputs of compressed gases can be realized in practice in this way. This is an obstacle to utilization of the process on an industrial scale.
EP 1,357,156 discloses a process for the preparation of a low-dust, microbeaded or microgranular, silane-modified oxidic or silicatic filler in a compressed gas. A disadvantage of this process is that the separator which is described as a constituent of the experimental apparatus and in which the compressed gas is decompressed does not adequately separate substances that are in the compressed gas and are extracted from the filler. This lack of separation leads to recontamination of the compressed gas in the course of the extraction. As a consequence, substances such as ethanol, are transported through and out of the separator and accumulate in other parts of the pressure apparatus, for example in the buffer tank.
Compressed or liquefied gas in the buffer tank can establish a solution equilibrium of the substances transported through and out of the separator. As a consequence, the compressed gas fed back to the pressure autoclave from the buffer tank is not substance-free, and this results in a reduced uptake capacity (loading capacity) of the compressed gas in the extraction tank which, in turn, results in a lower substance uptake of the compressed gas in the pressure autoclave. The reduced loading capacity of the compressed gas, for example for ethanol, therefore results in prolonged extraction times for the removal of substances from the silane-modified filler. This makes the procedure unattractive for use on an industrial scale.